image background transparent

I found this image of a scroll on the internet. It's got a transparent background. I mean, if you use it on a webpage with a black background you don't see a white square behind the scroll.
I'd like to write some words on the scroll and then use it on my webpage, but when I edit the picture and save it, the background is not transparent aymore. I don't have any fancy image editing programmes on my computer unfortunately. I used paint first, then tried irfan view, same result. Does anyone know how I can do this without losing the transparency of the background? Do I need a certain programme for this?